1. Field of the Invention
The present invention relates to a method and system for controlling an automatic transmission, to perform hydraulic pressure control to various frictional engagement elements at the time of gear shifting.
2. Description of Related Art
Conventionally, an automatic transmission for a vehicle selects a predetermined gear by selectively hydraulically actuating various frictional engagement elements (clutches, brakes, and the like) of a gear ratio change mechanism and shifts the gear from one to another by changing operation of a frictional engagement element.
When working hydraulic pressure to the selected frictional engagement element is too high at the time of gear shift, transient engagement capacity of the frictional engagement element (transmission torque capacity) becomes excessive and a large gear shift shock occurs. On the contrary, when the working hydraulic pressure is too low, transient transmission torque capacity becomes too small, slip of the frictional engagement element increases and the life of the automatic transmission is shortened. Consequently, it is necessary to properly control the working hydraulic pressure.
Japanese Patent Publication No. 5-81793 teaches a disengagement control for frictional engagement elements. In this control, during a period until start of an effective gear shift, an instruction value for hydraulic pressure of a clutch to be disengaged is reduced at a predetermined change rate (gradient) as shown in FIG. 16.
Since the hydraulic pressure is decreased in order to detect a proper hydraulic pressure value, the gradient cannot be set steep. The pressure reduction time required to attain a predetermined hydraulic pressure varies between the cases where the hydraulic pressure is relatively low and high. As a result, the response time from the generation of the instruction of gear shift until the start of the effective gear shift varies. Therefore, for instance, in case of a kick-down gear shift for an acceleration, down-shift occurs only slowly or response time varies from vehicle to vehicle.
Japanese Patent Laid-Open No. 8-159266 (U.S. Pat. No. 5,643,124) teaches a timing control for an engagement side frictional engagement element. In this control, the timing of control is advanced as the vehicle speed is lower in consideration of a response delay from a hydraulic pressure instruction.
In this control as well, however, the timing of controlling the frictional engagement element on the engagement side is influenced by the variation in the hydraulic pressure characteristics in a manner similar to the control of the disengagement side frictional engagement element. That is, when the pressure reduction gradient is too steep for the instruction value, for example, as shown in FIG. 17, even if the start of the effective gear shift is detected and the pressure reduction instruction is canceled with the decrease in the hydraulic pressure in the disengagement side frictional engagement element, the hydraulic pressure is reduced more by an amount corresponding to the response delay.
Even in the case where there is a variation such that the hydraulic pressure applied in response to the instruction value is lower than the design value, as shown in FIG. 18 for example, the start of the effective gear shift cannot be detected also during the response delay time for the pressure reduction operation from the frictional engagement element, so that the pressure reduction is executed. Consequently, pressure reduction is carried out more than necessary.
In case of using a hydraulic control mechanism for generating hydraulic pressure at the gear shift transient time by also using an accumulator, the lower the pressure is, the larger the degree of occurrence of excessive pressure reduction is due to the operation delay of a piston for regulating the operation of the accumulator or the like.